1. Field of the Invention
The invention relates to a method for manufacturing a composite strip, coated with a tin-silver alloy, for producing electrical contact components.
2. Description of Related Art
Tin-silver is a very good contact material. It is characterized principally by its low electrical resistance, its hardness, and its abrasion resistance.
The possibilities for coating an electrically conductive base material with a tin-silver alloy by electroplating are, however, limited. U.S. Pat. No. 5,514,261 discloses in this connection a way to deposit a silver-tin alloy by electroplating from a cyanide-free bath. The bath is prepared using silver as the nitrate or diamine complex, tin as a soluble tin(II) or tin(IV) compound, and mercaptoalkane carboxylic acids and sulfonic acids. Films of silver-tin alloys with a silver content of approximately 20 wt % to 99 wt % can be deposited from this bath.
The silver concentration of a coating manufactured in this manner is relatively high; films with lower silver concentrations cannot be attained. In addition, the film generated by electroplating is finely banded, with a slight micro-roughness. The film is brittle, and will tolerate only small bending stresses.
It is the object of the present invention to provide a method which makes possible the manufacture of a high-quality tin-silver coating on an electrically conductive base material.
In accordance with the present invention, these and other objects are achieved by the following process. A film made of tin or a tin alloy is first applied onto a starting material made of an electrically conductive base material. A film of silver is then deposited thereonto. Copper or a copper alloy is preferably used as the base material. The tin film can be applied in the molten state, and the silver film by electroplating. Both the tin film and the silver film can also be deposited by electroplating. A further alternative provides for manufacturing the tin film in the molten state and the silver film by cathodic sputtering. The diffusion operations which occur in the coating result in a homogeneous film of a tin-silver alloy. This formation can be assisted by heat treating the composite strip.